Nonlinear Dynamics of a Hysteresis Western Boundary Current Perturbed by a Mesoscale Eddy at a Gap with an Island

Abstract

We study a hysteresis western boundary current (WBC) flowing across a gap impinged by a mesoscale eddy, with an island of variable meridional size in the gap, using a 1.5-layer ocean model. The hysteresis curves suggest the island with a larger size facilitates the WBC intrusion by shedding eddy more easily. Both anticyclonic and cyclonic eddies are able to induce the critical WBC transition from penetration regime to leap regime, and vice versa. The vorticity balance analysis indicates increased (decreased) meridional advection induces the critical WBC shifting from the eddy shedding (leaping) regime to the leaping (eddy shedding) regime. The meridional size of the island significantly affects the critical WBC transition in terms of the critical strength of mesoscale eddy. The regime shift from penetration to leap is most sensitive to the eddy upstream of the WBC for small islands, and most sensitive to the southern anticyclonic eddy and northern cyclonic eddy for moderate and large islands. It is least sensitive to the central cyclonic eddy for small islands and to the cyclonic eddy upstream of the WBC for moderate and large islands, and to the northern anticyclonic eddy regardless of island size. The regime shift from leap to penetration is most sensitive to the cyclonic eddy upstream of the WBC and to the northern anticyclonic eddy. It is least sensitive to the anticyclonic eddy from the south, and the least sensitive location of the cyclonic eddy shifts northward from the gap center as the island size increases.

Influence of multiple islands on the hysteresis and dynamics of a western boundary current perturbed by a mesoscale eddy at a gap

“…Next, the presence of the multiple states and hysteresis of the WBC in a gap-leaping system was verified in laboratory experiments (Sheremet and Kuehl, 2007;Kuehl and Sheremet, 2009;Kuehl and Sheremet, 2014;McMahon et al, 2021). Moreover, the external factors, i.e., meridional wind (Wang et al, 2010), island in the gap (Mei et al, 2019), mesoscale eddy (Yuan and Li, 2008;Yuan and Wang, 2011;Mei et al, 2022), and largescale circulation in the SCS (Mei et al, 2023), are also proved to be crucial to affect the WBC path in a gap-leaping system. As for the other aspects, Wang and Yuan (2012); Wang and Yuan (2014) studied the collision of two opposite WBCs with equal or unequal transport at a gap, and Song et al (2019) and McMahon et al (2021) studied the effect of throughflow on the WBC transition.…”

“…In the vicinity of the Luzon Strait, the westward-moving mesoscale eddies from the northwestern Pacific interact with the WBC and frequently induce the variation of the WBC path (Yuan et al, 2006;Yuan and Li, 2008;Sheu et al, 2010;Yuan and Wang, 2011;Zhong et al, 2016;Yang et al, 2020;Mei et al, 2022). A reduced-gravity model was used to study the interaction of an eddy with a WBC by Chern and Wang (2005) and Kuo and Chern (2011); however, a gappy western boundary was not considered in their idealized model.…”

“…On the other hand, Yuan and Wang (2011) focused on the WBC transition perturbed by a mesoscale eddy in a gap-leaping system and found that both cyclonic and anticyclonic eddies could induce the critical-state WBC shifting regimes from penetration to leap, while only cyclonic eddies could induce the WBC transition from leap to penetration. Mei et al (2022) considered the effect of an island with variable meridional sizes on the eddy-WBC interaction. They found that both anticyclonic and cyclonic eddies could shift regimes of the critical-state WBC from penetration to leap, and vice versa.…”

“…Metzger and Hurlburt (2001) found that the Calayan Bank and a shoal north of the Calayan Island within the Luzon Strait reduce the westward intrusion of the Kuroshio into the SCS by using the NRE Layered Ocean Model. Mei et al (2019); Mei et al (2022) investigated the influence of an island on the WBC regime and found that the presence of an island promotes the WBC shedding eddies compared with the no-island case in the gap. Previous numerical studies focus on the effect of one island in the gap (resembling the central Batanes Island), which facilitates the WBC intrusion usually.…”

“…To what extent the combined quantitative impacts the southern and central islands on the WBC hysteresis and the critical-state WBC transition is worth studying. This paper is a continuation of Mei et al (2019); Mei et al (2022) and further considers the eddy-WBC interaction in the vicinity of a gap between two islands, which resemble the central Batanes Island and the southern Babuyan Island in the Luzon Strait.…”